P.L. Walker

Nature of the porosity in American coals

Abstract The nature of the porosity in a number of 40 × 70 (i.e. − 40 + 70) mesh size American coals, varying in rank from anthracite to lignite, has been studied using the following characterization techniques: gas adsorption, helium and mercury displacement, and mercury porosimetry. Surface areas calculated from carbon dioxide adsorption at 298 K are consistently higher than those calculated from nitrogen at 77 K, indicating the molecular sieve character of the coals. Total pore volumes have been measured in the diameter range 12–29 600 A and then divided into macropores (300-29 600 A), transitional pores (12–300 A), and micropores (4–12 A). Coals vary widely in their total pore volumes (porosities range between 4.1 and 23.2%). In the lower-rank coals (carbon content less than 75%), porosity is primarily due to the presence of macropores. In coals having a carbon content in the range 76–84%, about 80% of the total open pore volume is due to micro and transitional pores, whereas in the coals of higher carbon content microporosity predominates. Coals having about 35–55% of their total open pore volume in the transitional range are expected to be most suitable for use for adsorption of organic molecules from solution. Thus, they are of interest as possible materials to be used in water purification. In general, these results are considered to be of importance in understanding the extent and rate of interaction of coals with gases and liquids.

Gas Reactions of Carbon

Publisher Summary This chapter reviews the majority of pertinent papers on gas–carbon reactions. The inherent chemical reactivity of the carbon and mass transport of the reactants and products can play an important role in affecting the kinetics of gas–carbon reactions. The chapter discusses the possibilities of using bulk-density and surface-area profile data on reacted carbons for better understanding of reaction mechanisms. One of the steps involved in a gas–carbon reaction is the chemisorption of the gas on the carbon surface. Some of the products of the gas–carbon reactions chemisorb on the carbon surface under certain conditions. Therefore, an understanding of the chemisorptions of gases on carbon is essential for the understanding of the gas–carbon reactions. In chemisorption, it is known that the surface atoms must have free valence electrons to form strong chemical bonds with gas molecules or atoms. Much recent work using electron paramagnetic resonance absorption techniques has confirmed the presence of unpaired electrons in various types of carbons. The number of unpaired electrons is a complex function of carbon heat-treatment temperature, apparently being affected primarily by the number and nature of imperfections in the carbon structure.

Laser raman studies on carbons

Laser Raman spectra were studied of natural graphite (SP-1) and carbonaceous materials including pyrolytic graphite, carbon black, glassy carbon, coal, “white” carbon and sputtered carbon. All of these carbons have two Raman bands at 1580 cm−1 and 1360 cm−1, except for natural graphite which has a single sharp Raman band at 1580 cm−1. The relative intensity of the 1360 cm−1 band to the 1580 cm−1 band and the half band width increase going from graphite through glassy carbon to carbon black. The 1360 cm−1 band in glassy carbon becomes sharper and stronger with the increase of heat-treatment temperature (HTT), while the addition of iron to the glassy carbon matrices results in a decrease in intensity and half band width of this band with increasing HTT and iron content. Sputtered carbon and “white” carbon, prepared from graphite irradiated by a high power laser, showed an additional broad band around 2140cm−1. This band is believed to originate from conjugated acetylenic bands (—CC—)n.

Reactivity of heat-treated coals in air at 500 °C

Abstract Twenty one US coals, of widely ranging rank, have been carbonized under controlled conditions to 1000 °C, and the reactivity in air at 500 °C of the resulting chars or cokes has been measured by a gravimetric method. The reactivities lie within a well-defined band when plotted against rank of the parent coal. The lower-rank coal chars are more reactive than those prepared from high-rank coals. In extreme cases, the reactivity found for a Montana lignite char is some 100 times as great as that obtained for a char produced from a Pennsylvania low-volatile coal. Variation of reactivity with heat-treatment temperature (600 to 1000 °C) has been studied for three coals. As heat-treatment temperature increases, there is a decrease in reactivity. Some results are reported on the effects which mineral matter and pore structure have on the reactivity parameter. Chars containing high concentrations of magnesium and calcium impurities are most reactive. The amount of macro and transitional porosity in a char has a marked influence on reactivity.

Reactivity of heat-treated coals in carbon dioxide at 900 °C

Abstract Reactivities of sixteen 40 × 100 (U.S.) mesh U.S. coals charred to 1000 °C were measured in carbon dioxide at 900 °C. Chars derived from coals with less than 80% carbon, on a dry-ash-free basis, were the most reactive. These chars also gave the widest spread in reactivity. Plots of inorganic element content in the chars versus reactivity showed that magnesium and calcium are important to char reactivity. Six coals were acid-washed with hydrochloric acid and four coals were further demineralized with hydrofluoric acid. Most acid-treated coals showed a decrease in reactivity; but two coals of high rank increased in reactivity. This increase in reactivity is attributed to the creation of additional porosity as a result of mineral matter removal and thus a reduction in resistance to carbon dioxide diffusion to reactive sites. Two demineralized and two original coals were divided into four size ranges and chars were produced from each size of each coal. Gasification rates increased monotonically with decreasing particle size reacted.

CO hydrogenation over well-dispersed carbon-supported iron catalysts

It became apparent that carbon-supported iron systems were quite interesting and a broader base was required for comparison, so catalysts composed of iron dispersed on graphitized carbon, carbon blacks, and activated carbons were prepared. In contradiction to earlier studies of Fe/carbon CO hydrogenation catalysts, these carbon-supported iron samples were very active, and they typically had higher activities and olefin/paraffin ratios than unpromoted Fe/Al/sub 2/O/sub 3/ catalysts. In addition, an aqueous impregnation technique was developed which allowed the preparation of very highly dispersed iron on certain carbons. These iron/carbon catalysts could be placed in two general groups: one with high dispersion (HD) and one with low dispersion (LD). These reduced catalysts exhibited extremely interesting chemisorption behavior at 300 K - hydrogen chemisorption either did not occur or was very low while CO chemisorption always occurred, with very large CO uptakes existing in some cases indicating the presence of very small iron particles (i.e., high dispersions). In addition to these chemisorption properties, significant differences existed in catalytic behavior which could be correlated with the crystallite size of the iron. This note describes these variations in catalytic properties.

Measurement of interlayer spacings and crystal sizes in turbostratic carbons

Abstract Interlayer spacings and crystal sizes of turbostratic carbons are frequently obtained by the direct and improper application of the Bragg, Scherrer, and Warren equations to the experimental X-ray scattering profile. The components of this scattering profile and their sources are considered, and the applicability of the Scherrer and Warren equations to turbostratic carbons is discussed. An analytical method is outlined for separating the incoherent scattering, the scattering due to crystallographically amorphous atoms and single graphitic-type layers, and the 002, 004, 10, 11 and 20 bands from the experimental data. Proper application of the Bragg, Scherrer, and Warren equations to these bands yields useful and accurate values of the interlayer spacings and crystal sizes. It is shown that these values differ significantly from the results obtained directly from the experimental profile.

Enhancement of lignite char reactivity to steam by cation addition

Abstract Chars produced from lignites typically have much higher reactivities to gasification than those produced from bituminous coals. This has been attributed previously to the presence of carboxylate salts of inorganic constituents on the lignites. Upon charring of the lignites, the carboxylate salts decompose leaving behind well dispersed inorganic constituents which act as catalysts for gasification. In this study, a raw lignite has been treated with HCl and HF to demineralize it and to increase its carboxyl content prior to exchanging selected cations with the hydrogen on the carboxyl groups. Up to 2.14 mmol of calcium per g of coal could be added using this procedure. Addition of varying amounts of calcium to the lignite resulted in the production of chars containing calcium contents ranging from 1.1 to 12.9 wt %. Such addition resulted in a rectilinear increase in reactivity of the char to steam with increasing amount of calcium added. Maximum reactivity attained was over ten times the reactivity found for the char produced from the raw lignite. At comparable molar loadings of metal cations onto the acid-treated lignite, the chars subsequently produced had reactivities in steam in the order: K >Na ≈ Ca >Fe >Mg. Char reactivity could also be enhanced by the addition of cations to nitric acid-treated char which had been produced, in turn, from demineralized lignite.

6Å molecular sieve properties of saran-type carbons

Abstract The results of a molecular probe study on two Saran carbons prepared up to 1000°C and on polyvinylidene chloride carbons prepared up to 1500°C are reported. The Saran carbons adsorb appreciably more isobutane than neopentane over the whole carbonization range, indicating 6A molecular sieve properties. Benzene and cyclohexane are also adsorbed to an appreciably greater extent than neopentane. Polyvinylidene chloride carbons prepared at comparable temperatures adsorb neopentane freely; neopentane molecular sieve effects are only observed for samples heated above 1200°C. It is concluded that the observed molecular sieve effects are due to slit-shaped pore constrictions having a size between ca. 4·5 and 5·7A in thickness and connecting cavities of at least ca. 12A in thickness.

Carbon as a support for catalysts: I. Effect of surface heterogeneity of carbon on dispersion of platinum

Differences in the degree of dispersion of platinum supported on a graphitized carbon black, subjected to varying levels of carbon burn-off in air, have been found. Dispersion increases with the extent of prior gasification of the carbon support. This increase cannot be explained in terms of total surface area increase of the carbon support as a result of gasification. Rather gasification increases the surface heterogeneity of the carbon support, which in turn increases the potential energy barrier for the diffusion of platinum species across the carbon surface during sample preparation which uses temperatures up to 500 °C.